1. Field of the Invention
The present invention relates to an improvement of a brake control system, in particular to an electronic control system for a brake system which detects a human brake operation input quantity such as a brake-pedal stroke and electronically controls a braking effect to obtain appropriate deceleration of a car speed according to the detected human operation input.
2. Description of the Prior Art
It is to be noted here that the term "deceleration" means a rate of decrease of speed of a motion, in other words, indicates a value of negative acceleration.
Conventionally, disclosed is an electronic control brake system, for example, in the Japanese Patent Publication SHO 56-033254 in which a constant deceleration of a car speed is achieved in response to a specified human operation input quantity for braking without changing a braking effect regardless of increase or decrease of a carrying load on a car, a frictional factor of a brake friction member, and the like.
However, the above-mentioned conventional control system has a difficulty in controlling the braking effect when driving a car on a slope. In more detail, there is a case where a certain braking force is required for obtaining a constant speed when driving a car on a downward slope. Even when a braking operation is effected to generate a braking force for obtaining a constant speed, a brake fluid pressure is automatically so controlled as to generate undesirable deceleration corresponding to the braking operation input quantity, resulting in failure in achieving a car running at a constant speed due to the generated deceleration.
For example, when a car is running down a slope at an acceleration of 0.6 .alpha. (hereinafter, ".alpha." denotes a constant value of a positive acceleration), if a brake pedal stroke corresponding to a deceleration of 0.2 .alpha. is effected in order to decrease the acceleration from 0.6 .alpha. to about 0.4 .alpha., an abrupt braking force is generated to abruptly reduce the car speed at a deceleration of 0.2 .alpha. (i.e., acceleration of -0.2 .alpha.), which results in an awkward braking operation.
In view of the drawbacks mentioned above, the Japanese Patent Laid-Open (Unexamined) HEI 3-164360 discloses a brake system in which a braking force is so controlled as to obtain a predetermined deceleration or acceleration of a car speed as shown in FIG. 4. In more detail, when a human operation input quantity for braking is in a range larger than a specified value L.sub.0, a predetermined deceleration of a car speed is achieved as shown by a real curved line in FIG. 4, and on the other hand, when a braking operation input quantity is smaller than L.sub.0, the braking force is so controlled or so released as to permit an acceleration within a predetermined value as shown by a broken curved line in the figure, where the value L.sub.0 is so specified as to control the deceleration (or acceleration) to be 0 (zero).
However, since there is no consideration for the running conditions of a car before effecting a braking operation in the above-mentioned control system, the following disadvantage takes place.
In particular, when a further deceleration is desired to be added by stepping the brake pedal while a car is running up a slope at a deceleration of 0.6 .alpha. (i.e., acceleration of -0.6 .alpha.), no braking effect is generated at an initial state, in other words, the deceleration of 0.6 .alpha. of the car speed is maintained in the initial stage of the braking operation so long as the pedal stroke is in the range smaller than that corresponding to the deceleration of 0.6 .alpha.. When the pedal stroke exceeds the value corresponding to the deceleration of 0.6 .alpha., the further deceleration is then effected for the first time. Therefore, no change of the deceleration of the car speed takes effect in the initial stage of stepping the brake pedal to result in generating no braking effect. The fact that the operation of stepping the brake pedal takes no effect in the initial stage is not only undesirable but also even dangerous.